Enrichment of viable bactoeria in a micro-volume by free-flow electrophoresis

This paper essentially presents a new way to enrich bacteria with free-flow electrophoresis so that you can have a macro to micro interface for microfluidic analysis of samples in clinical settings. The paper describes a little bit how they use phaseguides to set up the experiment, but that does not seem to be the focus, so relation the phaseguide paper is not overwhelming.

Free flow electrophoresis works as follows:

  • bacteria suspended in buffer are injected in a continuous flow
    • surface of bacteria contains among others carboxylate, phosphate, and amino groups that are negatively charged in a pH range of 6–9
  • an electric field ${\vec E}$ is applied perpendicular to the flow direction
  • a negatively charged bacterium entering the chamber is subjected to a drag-force in the y-direction and an electrophoretic induced force in the x-direction resulting in a deflection from the initial flow direction towards the anode
  • Degree of deflection $\theta$ depends on relation of electrophoretic velocity ${\vec v_c}$ and the hydrodynamic velocity ${\vec v_h}$ of the bacterium
$$ tan \theta = \frac{\vec v_c}{\vec v_h}$$
  • Hydrodynamic velocity varies with position of bacteria in the chamber (parabolic flow profile)
  • Electrophoretic velocity is the product of the electric field strength and electrophoretic mobility of the bacteria ($\mu$)
$$ {\vec v_c} = \mu {\vec E}$$
  • Constant electrophoretic trapping at the gel interface can be obtained by keeping the deflection angle constant.
  • Governed by:
$$ tan \theta = \frac{\mu {\vec J}}{\sigma {\vec v_h}}$$ $$ {\vec J} = \sigma {\vec E}$$
  • here $J$ is current density and $\sigma$ is the conductivity of the medium
  • Picture!

Chip Design

  • The diagram pretty much says it all:

Bacteria

  • Used Gram-positive bacterial strain *Enterococcus faecalis Symbioflor 1
  • Achieve $~10^6 \text{ CFU ml}^-1$

Results

  • A 75% optimal increase in trapping is observed at 250 $\mu A$ current
  • Some other results, but basically it worked
Written on October 13, 2014